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HOW DOES A CHECK VALVE WORK?A GUIDE TO THE 8 STAGES OF CHECK VALVE OPERATION

PLASTIC • ONE-WAY • IN-LINE • SPRING-LOADED

For over 30 years, Smart Products USA, Inc.

has focused on the intricate details of how

one-way check / pressure relief valves work

in low pressure, low flow devices.

Sharing this knowledge with existing and

potential customers can save them time

and money as they develop their current or

future applications.

CHECK VALVE OPERATIONINTRODUCTION:

SMARTPRODUCTS.COM • 1-800-338-0404

VIEW 1:40 MIN VIDEO OF CHECK VALVE OPERATION AT

SMARTPRODUCTS.COM

AT SMART PRODUCTS USA:

All check / pressure relief valves are customizable by 9+ plastic body materials, 7+ o-ring materials, and opening pressure ranges from 0.09 – 20 PSI.

PLASTIC OPTIONS: Polypropylene, Nylon, Acrylic, Acetal, PVDF, Polycarbonate, Ultem®, ABS, Radel®

O-RING OPTIONS: Buna-N, EPDM, Viton®, Silicone, Fluorosilicone, Aflas, Kalrez®

As a resource, this white paper answers

critical questions, such as:

• Will a certain step affect the device’s

performance?

• Is there a part in the operation

process that will be critical to the

application?

This guide describes the check valve opera-

tion process in eight stages.

STAGES OF CHECK VALVE OPERATION

1 STAGE 1: SHUT

2 STAGE 2: OPENING

3 STAGE 3: OPEN

8 STAGE 8: RESET

4 STAGE 4: FULLY OPEN

5 STAGE 5: THE DESCENT

6 STAGE 6: TOUCH DOWN

7 STAGE 7: RESEALING


During the first stage, the valve remains shut and

prohibits flow in either direction. The valve is not

inactive; there is a lot of action taking place because

the laws of physics continue to rule over the valve in

what can be described as passive and active states.

1 STAGE 1: SHUTTHE ACTIVE & PASSIVE STATES OF A CLOSED CHECK VALVE

OVERVIEW: THE 8 STAGES OFCHECK VALVE OPERATION

PLASTIC BODY

SPRING POPPET

O-RING


A CLOSED, BUT ACTIVE VALVE

Passive

All valves begin in a passive state. They are not yet in

contact with any liquids or gasses, but the valve’s com-

ponents – plastic body material, o-rings, poppet, car-

tridge, and springs – remain in contact with each other.

The laws of physics win again if certain body materials

and o-rings remain together for long periods of time,

and two factors should be considered in any application:

1) leak rate and 2) stiction. Leak rate can be easily under-

stood when looking at a bicycle tire. If the tire loses 1 PSI

over one week, it has a leak rate of 1 PSI per week.

The next potential issue during the passive state is

stiction. Again, if a valve is inactive and sits for a long

period of time, the o-ring can stick to the o-ring seat and

cause an elevated opening pressure.

Stiction can also occur if soft o-rings are used. Aware-

ness of these two issues is the first step to ensure a

valve’s optimal performance when evolving from the

passive to the active state.

Active

Once the valve makes contact with a liquid/gas, it

becomes active and will never return to the passive

state. While the valve is still closed, the o-rings play a

key role when they first encounter the liquid/gas for the

following reasons:

• Certain chemical components can make the

o-ring expand

• Certain liquids, such as water, can contain

debris that will not allow the o-ring to seat

correctly

Application Environment

A final consideration is an application’s environment

because it can also affect the passive and active states

of a closed check valve.

• Will the valve reside in tubing?

• Will it be external or exposed to elements such

as temperature, humidity, sunlight, etc.?

The answers to these questions could affect a valve’s

performance based on the selected plastic body mate-

rial and o-rings. Additionally, to ensure optimal valve

performance every time, it is important to consider the

following:

• Shelf life of a valve

• All liquids / gasses coming into contact

with the valve

• Application environment

1 STAGE 1: SHUT (CONTINUED)



We are checking for tiny changes in pressure to assure our valves are seal-ing well. We test our valves for leak rate in the check direction. If the valve can hold pressure to our stringent requirements, it is accepted. To ensure maximum performance, we perform a leak test on 100% of our valves.

Stage 2 of check valve operation is a balancing act

between being shut in Stage 1 and open in Stage 3. A

check valve’s nature does not allow them to be switched

on and off, and this stage is notable when building a

custom valve for a specific application design criterion.

Once pressure has increased just enough to counter the

force of the spring holding the o-ring against the seat,

a small amount of flow will escape. Depending on the

application, this stage can be viewed one of two ways:

as an insignificant or significant amount of flow. A key

question to consider at this stage:

• Will the initial opening of a valve effect

my application?

The answer will help to determine which spring pressure

and o-ring combination are ideal in order to control flow

between Stage 2 (barely open) and Stage 3 (open).

2 STAGE 2: OPENINGTHE “IN-BETWEEN” OF SHUT & OPEN VALVES

GRADUAL INCREASE OF LIQUID/GAS



Our team has developed extensive knowledge about o-rings and how they perform in our valves. This information allows us to have some control over a valve’s performance by:

• Washing o-rings twice to remove any debris• Curing o-rings to affect their properties via various methods (i.e. sulfur, peroxide, etc.)• Prescribing a soak test (if needed) to determine chemical compatibility• Selecting o-ring materials and durometer (hardness of rubber), as well as spring force to be an

ideal match for a specific design criterion

All of the above solutions are available to customers, upon request. O-rings are available in 7+ materials. We also offer an o-ring’s durometer on a scale of 50 (soft) to 80 (hard). Available durometer will vary for each o-ring material.

Ideal environment for o-rings and the cartridge seat varies depending on how the valve is being used. However, we customize components to optimize a valve’s performance for every customer’s unique application requirements. Remember, no valve is completely leak free because of their physical properties, but we test 100% of all valves to ensure they seal.

O-rings & the Cartridge Seat

Another consideration of Stage 2 is to understand the key

role of o-rings and the cartridge seat (a.k.a. poppet). To

the naked eye, o-rings appear perfectly smooth. However,

by nature, they often have tiny imperfections that can,

over time, allow flow to escape. Once o-rings are selected,

they are wedged into the cartridge seat to create a seal.


Customers can choose an opening pressure to suit their design criteria. Valves are rated at the pressure where the “open” stage occurs.

We use specialized equip-ment to measure the effects of pressure on a valve’s perfor-mance including the opening PSI assignment. During this test, pressure builds up within the test volume until the valve opens. The valve opens far enough to allow a large amount of flow to escape, which causes a sudden drop in pressure. The test equipment records the point of this occur-rence. All Smart Products USA valves are rated in this manner.

Opening pressure ranges for Smart Products USA valves are 0.09 to 20 PSI.

Stage 3: Open involves a significant chain reaction

of events:

• Pressure builds

• The o-ring lifts off the cartridge seat

• A significant amount of liquid/gas flows

through the valve

After this series of events, the valve’s opening

pressure is assigned.

OPENING PRESSURE IS ASSIGNED3 STAGE 3: OPEN

SIGNIFICANT AMOUNT OF LIQUID/GAS


FOR CUSTOMERS LOOKING FOR RELIEF VALVES: THIS STAGE IS A CRITICAL STEP TO UNDERSTAND BECAUSE AN ACCURATE OPENING PRESSURE ENSURES A DEVICE WILL FUNCTION EXACTLY TO APPLICATION REQUIREMENTS.

During stage 4, the pressure moves fluid through the

valve with such velocity that the poppet travels to its

limit. At this point, the valve has achieved a FULLY OPEN

state. Once the valve is fully open, more flow can still be

achieved, but at a diminishing return because the poppet

cannot travel any further.

Note: The minimum pressure (at which flow is possible)

will surprisingly occur at a pressure lower than the initial

opening pressure. This outcome happens because more

pressure is required to overcome stiction, which can

occur if a valve is inactive and sits for a long period of

time.

REGULATED PRESSURE OCCURS4 STAGE 4: FULLY OPEN

THE POPPET GOES THE DISTANCE



We have found that by applying a parylene coating to o-rings, they are able to remain idle longer without the risk of an elevated opening pressure. We test every series of our valves to establish performance in liquid and gas applications begin-ning with OPENING, progress-ing through OPEN or minimum regulated pressure (SSRP), and ending with RESEALING.

FOR CUSTOMERS WHO WANT TO MAINTAIN A LEVEL OF PRESSURE IN APPLICATIONS, BEING AWARE OF THIS STAGE IS IMPORTANT. ONCE FULLY OPEN IS REACHED, A LARGE PRESSURE INCREASE WILL BE REQUIRED TO MARGINALLY RAISE THE FLOW. THAT IS WHY IT IS ALWAYS GOOD TO MAKE SURE THE VALVE YOU ORDER HAS SUFFICIENT FLOW AT YOUR OPERATING PRESSURES. THERE ARE NO OPERATIONAL PENALTIES FOR A LARGER VALVE OTHER THAN YOUR SPACE CONSTRAINTS.

PRESSURE DECREASES5 STAGE 5: THE DESCENT

When the valve’s internal components (the poppet,

spring, and o-ring) continue to travel, they eventually

make contact with the o-ring seat, also known as Stage 6:

Touch Down. The valve will not seal well at this stage

because there is no force pressing the o-ring back into

the seat. As mentioned in Stage 2: Opening, o-rings ap-

pear perfectly smooth, but they can have tiny imperfec-

tions, and it is through these ridges and gaps that flow

escapes. Therefore, the valve will not fully close until

Stage 7: Resealing. POPPET, SPRING, & O-RING TRAVEL BACK

6 STAGE 6: TOUCH DOWNINTERNAL COMPONENTS BEGIN TO MOVE BACK

The poppet plays a key role during these stages

because it is the connection between the spring and

o-ring seal. The poppet sets the spacing between these

two components. Thus, it directly contributes to the

actual pressure the valve operates at during all stages.

FLOW OF LIQUID/GAS DECREASES


Stage 5: The Descent is when pressure in an application

decreases, and the spring begins to pull the poppet

back towards the shut or resealing position. Watching

the process closely, the valve’s poppet begins to shut

following a different path. Note: the valve does not

instantly return to the shut stage – a slight delay, hyster-

esis, occurs due to internal friction or a change in forces

acting upon the valve. This lag does affect an applica-

tion’s performance, but for most users it is so subtle that

it is not even detectable. The journey back to resealing

is not instantaneous. These factors make a valve have a

difference in their opening pressure and reseal pressure.

In fact, the reseal pressure is around 20% below the

opening pressure.


The more we know about your specific pressure requirements, the more precise your check valve’s performance will be. To ensure accurate and reliable flow control, all our spring-loaded valves have been extensively tested to determine their open, regulated, and reseal pressures.

INTERNAL COMPONENTS BACK IN PLACE

Once the valve reseals, it is in Stage 8: Reset. The valve

has returned to its original position and is ready to go

through all eight stages of operation again.

VALVE IS READY TO REPEAT ALL STAGES OF OPERATION AGAIN

8 STAGE 8: RESET

7 STAGE 7: RESEALING

O-RING IS ON SEAT AND SEALS

BACK TO ORIGINAL POSITION


In Stage 7: Resealing, the o-ring continues its journey

back to the seat. Previously, the o-ring was only touching

the seat, but now the spring force (via the poppet) has

compressed the o-ring enough to hold it into place and

create a seal against every gap.

THIS STAGE IS PARTICULARLY NOTEWORTHY FOR CUSTOMERS WITH APPLICATIONS THAT CANNOT DROP BELOW A CERTAIN PRESSURE BECAUSE RESEAL PRESSURE WILL ALWAYS BE LOWER THAN THE OPENING PSI. FOR EXAMPLE, IF YOUR DEVICE REQUIRES A RESEAL PRESSURE OF 10 PSI, THEN YOU WILL WANT A VALVE WITH AN OPENING PRESSURE AROUND 12 PSI. IN ADDITION, RESEAL PRESSURES UNDER 1 PSI MAY NOT BE REPEATABLE DUE TO THE LOW SEAT PRESSURE.

CHECK VALVE OPERATION

ABOUT SMART PRODUCTS USA, INC.

SMARTPRODUCTS.COM/CHECK_VALVE_OPERATION.PHP

VIEW 1:40 MIN VIDEO AT:


Smart Products USA, Inc. designs and manufactures

check and pressure relief valves, JACO Smart Fittings,

liquid and air pumps, and related components.

Our products provide accurate, reliable flow control

in advanced, low pressure, low flow systems.

An ISO 9001:2008 Certified Company.

644 FANNING FIELDS RD.MILLS RIVER, NC 28759

how does a - smart products · for over 30 years, smart products usa, inc. ... • selecting o-ring...

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